This invention relates to coherent imaging using vibratory energy, such as ultrasound, and, in particular, to ultrasound imaging which employs multi-element array transducers.
There are a number of methods in which vibratory energy, such as ultrasound, can be used to produce images of objects. The present invention relates to the reflection method for producing ultrasound images in which a single ultrasonic transducer array is used for both transmission and reception of ultrasonic pulses. An image is produced in which the brightness of each pixel is a function of the amplitude of the ultrasound reflected from the object to the receiver.
Ultrasonic transducers for medical applications are constructed from one or more piezoelectric elements sandwiched between a pair of electrodes. When an appropriate voltage pulse is applied, the piezoelectric element emits an ultrasonic pulse into the medium. Conversely, when an ultrasonic pulse strikes the piezoelectric element, the element produces a corresponding voltage across its electrodes. A number of such ultrasonic transducer constructions are disclosed in U.S. Pat. Nos. 4,217,684; 4,425,525; 4,441,503; 4,470,305 and 4,569,231, all of which are assigned to the instant assignee.
When used for ultrasound imaging, the transducer typically has a number of piezoelectric elements arranged in an array and driven with separate voltages. By properly controlling the relative time delays of the applied voltages on each element, the ultrasonic waves produced by the piezoelectric elements can be made to combine to produce a net ultrasonic wave focused at a selected point. This focal point can be moved on each successive transmitter firing, so that the transmitted beams can be scanned across the object without moving the transducer.
Similar principles apply when the transducer is employed to receive the reflected sound. The voltages produced at the transducer elements in the array are individually delayed in time and then summed together such that the net signal is dominated by sound reflected from a single receive focal point in the subject. This summed receiver signal is often called the "beamsum".
For a wave at a single frequency f, it is well known that a shift in time .DELTA.t is equivalent to a shift in phase .DELTA..phi. through the relationship .DELTA..phi.=2.pi..phi..DELTA.t. The pulses typically used in ultrasound imaging contain a wide range of frequencies, so this equivalence is only approximate. Some ultrasound imaging systems use the approximate equivalence to combine time delays and phase delays to produce the desired focusing on transmit and/or receive. The process of applying time and/or phase delays to produced focused transmit and receive beams is often called "beamforming."
An ultrasound image is formed by making a series of reflection measurements in a set of desired directions. For each measurement, a focused ultrasonic wave is transmitted. Then the system switches to receive mode and the reflected ultrasonic wave is received, focused and stored. When a complete set of scan directions has been obtained, the ultrasound image is constructed and displayed, and the process then repeats for the next imaging frame. A number of such ultrasonic imaging systems are disclosed in U.S. Pat. Nos. 4,155,258; 4,155,260; 4,154,113; 4,155,259; 4,180,790; 4,470,303; 4,662,223; 4,669,314 and 4,809,184, all of which are assigned to the instant assignee.
The proper operation of an ultrasonic imaging system such as just described assumes that there is a known, constant speed of sound in the medium through which the ultrasonic pulses are conveyed. If the sound speed is not constant, sound pulses transmitted from certain elements in the array can arrive earlier or later than expected at the desired focal point and will not properly combine with the other pulses. As a result, the net transmitted wave will not be optimally focused. Similarly, on reception, the signals on each element in the array will not be delayed optimally before summing so that the receive focusing will be degraded. If the deviations from the assumed propagation times could be measured or estimated, the ultrasound image could be improved by correcting the applied time delays for the deviations.
The human body is known to consist of many different tissues with differing sound speeds. Despite this, in medical applications the assumption of constant sound speed produces good images on many patients. However the distribution of the various tissue types varies widely with patients, and some patients are only poorly imaged. The body wall, in particular, which consists of relatively thick muscle and fat layers with sound speeds significantly different from the average sound speeds of the internal organs, can degrade the image for some patients. There would be a substantial medical benefit if the images of these patients could be improved by correcting for nonuniformity in the sound speed in the body. Such time delay corrections may need calculation for each separate transmit-receive direction, since the sound speed nonuniformities may vary significantly with beam direction. The corrections may also require calculation on a real-time basis due to patient and transducer motion in clinical applications. U.S. Pat. No. 4,989,143, assigned to the instant assignee, discloses a method and system for correcting the time delay of the separate signals produced by the transducer array elements to account for sound speed variations. In this prior method the demodulated baseband signals from pairs of adjacent receiver channels are cross correlated over a range of echo samples and a phase error difference between the two channels is calculated. This phase error difference is accumulated across the array and applied as a time delay correction to the corresponding transmit and receive channels. However, under some circumstances, this method does not improve image quality. Specifically, the signals which are cross correlated correspond to ultrasound energy arriving on individual array elements. The individual array elements can have a significant response to reflectors far from the beam direction which is being corrected. The time delay estimates can be corrupted by these off-axis reflectors if the reflectors are sufficiently bright or if the transmit beam is sufficiently defocused. In addition, the accumulation of phase error differences to calculate the time delay corrections also accumulates the inaccuracies in each channel pair measurement, reducing the accuracy and robustness of the corrections.
U.S. Pat. No. 5,172,344, assigned to the instant assignee, also discloses a method and system for correcting the time delay of the separate signals produced by the transducer array elements. In this prior method the demodulated baseband beam signals produced at reference beam angles are cross correlated and the phase errors between the beams are calculated. These phase errors are calculated across the entire field of view and then Fourier transformed to produce time delay corrections which are applied to each transmitter and receiver channel. However, the determination of the time delay errors from the Fourier transform is based on an approximation, and care must be taken, in clinical applications, to avoid large time delay errors or patient motion between beam firings, which can make the approximation invalid.